Phase Difference resulting from Metallic Reflexion. 473 



Considering the accuracy with which interferometer 

 measurements could be conducted, it occurred to the writer 

 that this method may be used for experimentally determining 

 the phase difference resulting from metallic reflexion with 

 greater accuracy, since it is free from the defects of other 

 methods which suffer * from the unavoidable impurities of the 

 polished metal surfaces exposed to air. 



Before going into the theory of the method, a word may be 

 said about the experimental arrangements adopted to secure 

 the necessary photographs of the fringes for measurement. 



Fig-. '2. — Showing the types of changes resulting- from metallic reflexion 

 in polarized and unpolarized light. 



Light not polarised 



L i 9 ht polarise-' 



One of the 

 normal 

 band system 

 in air. 



After 

 metallic 

 reflection. 



In the plane 

 of 

 incidence. 



At 4-5° to 



the plane 



of 



incidence. 



Perpendicular 

 to the plane 

 cf 

 incidence. 











— 











The glass Lummer plate chemically silvered on the bottom 

 surface was taken, and half the silvering along its length 

 removed by gentle rubbing with dilute nitric acid, and 

 the glass portion thus showing cleaned and dried carefully. 

 It was then mounted in its usual stand on the Hilger inter- 

 ference spectroscope. Light from a mercury vapour lamp 

 was ibcussed on the slit by a pair of collimating lenses, 

 between which was interposed a 2 in. Mcol polarizer as illus- 

 trated in fig. a (PL VIII.) . The polarizer was set to polarize 

 the incident light in the plane of incidence, and thus cut off 

 the other component set of bands resulting from the metallic 

 reflexion and corresponding to the case of light polarized 

 perpendicular to the plane of incidence. Under these 

 circumstances, the collimated beam in passing through this 

 half metal and half air plate was split into two halves, 

 namely, the one that underwent metallic reflexion and the 

 other that did not, and the fringes due to these two were 

 obtained consecutively on the same plate, but with a natural 



* Drude's ' Theory of Optics/ p. 366. 



